In recent years, realization of ultrahigh-speed/high-capacity wireless communication has come into great demand, and utilization of a millimeter wave band is useful for realization of this type of communication. In particular, development of a broadband circuit element, which does not require a license and covers a 59- to 66-GHz band, is important. With this development, it is possible to realize an ultrahigh-speed wireless LAN, a home link, TV indoor wireless transfer, an inter-vehicle communication system and others at a transmission rate exceeding, e.g., 400 Mbps.
As such, an NRD guide has been conventionally used as a millimeter-wave or microwave transmission circuit. In this NRD guide, as shown in FIG. 11, a dielectric waveguide 101 formed of, e.g., polytetrafluoroethylene, better known as TEFLON®, having, e.g., a dielectric constant ∈r=2.04, is provided between a pair of parallel conductor plates 102a and 102b. A width of each of these conductor plates 102a and 102b, i.e., a height of the dielectric waveguide 101, is set to be less than a ½ wavelength of a frequency of a millimeter wave, and a width of the dielectric waveguide 101 is set to be approximately ½ wavelength. For example, if an operating frequency is 60 GHz, a height of the dielectric waveguide 101 is set to 2.25 mm and a width of the dielectric waveguide 101 is set to 2.5 mm. As a result, a millimeter wave having the operating frequency can be propagated through the dielectric waveguide 101, but the millimeter wave having the operating frequency cannot be propagated outside the dielectric waveguide 101, and hence the millimeter wave is trapped in and transmitted through the dielectric waveguide 101.
Although such an electric field in a cross section as shown in FIG. 11 is generated in an operating mode (an LSM mode) of the millimeter wave transmitted through this dielectric waveguide, an LSE mode which is an unnecessary parasitic mode is produced as shown in FIG. 12 when the dielectric waveguide 101 bends or branches between the pair of conductor plates 102a and 102b. 
In order to suppress this LSE mode, a mode suppressor 103 having a ¼ wavelength choke structure is inserted into the dielectric waveguide 101 in the prior art as shown in FIG. 15 (see Japanese Patent Application Laid-open No. 2000-341003).
However, inserting the above described conventional mode suppressor 103 into the dielectric waveguide 101, there arises a problem that requires a troublesome operation involving time and labor; namely, the dielectric waveguide 101 must be cut open in a longitudinal direction, and the mode suppressor 103 is inserted into and attached to this cut portion. Thus, the present inventors have discovered that arranging a conductor in the vicinity of or in close contact with the dielectric waveguide 101 can effectively control the LSE mode, which is a parasitic mode (see Japanese Patent Application No. 2003-49953).
However, where the dielectric waveguide 101 is brought into contact with the conductor, there is a problem wherein transmission characteristics may not be obtained as designed, and irregularities in the transmission characteristics become substantial.
In a circuit using an NRD guide, a microstrip line may be used in some cases, where coupling the NRD guide with the microstrip line through a coaxial line can reduce deterioration in the transmission characteristics. However, there is a problem that the transmission characteristics may not be obtained as designed in coupling the microstrip line and the coaxial line, and irregularities in the transmission characteristics again become substantial.
The NRD guide has excellent characteristics wherein transmission loss is very low in a millimeter-wave band as described above and radiation of an unnecessary millimeter wave is not generated at all in a bent part or a discontinuous part of the dielectric waveguide. The NRD guide is suitable for loading a two-terminal element such as a diode, but has a problem that it is not suitable for loading a three-terminal element.
On the other hand, the microstrip line is suitable for loading of a three-terminal element or the like, and can constitute various kinds of flexible circuits. However, the microstrip line has a problem that it demonstrates a large transmission loss in a millimeter-wave band.
Thus, there can be considered a hybrid structure in which the NRD guide is used for a transmission part and the microstrip line is used for a circuit element loading part, such as a three terminal element. However, there is a problem that the NRD guide and the microstrip line cannot be efficiently coupled.
In view of the above-described problems, it is an object of the present invention to provide an NRD guide transition capable of realizing, with a low loss, a hybrid structure in which an NRD guide is used for a transmission part and a microstrip line is used for a circuit element loading part, and provide a coupling structure of a dielectric material and a conductor capable of assuredly obtaining designed transmission characteristics with a simple configuration.